Byoung-Hoon Hwang, Pusan National University (Republic of Korea)
W.G. Ernst, Stanford University (United States)
Moon Son, Pusan National University (Republic of Korea)
Joon-Dong Lee, Pusan National University (Republic of Korea)
Late Cretaceous to Early Tertiary granitic rocks in the Gyeongsang Basin are distributed as numerous small stocks offset by several strike-slip faults. Based on field relations and petrography, they can be divided to six rock types in three groups; stocks of each group range in age from 80-50 Ma. Group I is composed of granodiorite, enclave-rich porphyritic granite, and enclave-poor porphyritic granite containing mafic microgranular enclaves (MME) and/or mafic clots, implying magma mixing or mingling. Group II consists of equigranular and micrographic granite, indicating emplacement at shallow depth; rocks of group II intrude those of group I. Group III, A-type alkali-feldspar granite, is characteristic of tensional tectonic settings.
SHRIMP-RG U-Pb dating of zircons from these rocks has documented two concentrated times of granitic magmatism. The first age shown in group I rocks reflects magma mixing due to subduction of Izanagi-Pacific ridge at 75-65 Ma. The host granite and included MME possess the same ages, 75.0±0.5 and 74.9±0.6 Ma, respectively. The second age is mainly related to A-type granites of group III emplaced into a tensional tectonic environment at 55-50 Ma. Two A-type granites in the Gigye and Gyeongju areas along the Yangsan fault showing same ages, 53.9±0.3 and 53.6±0.7 Ma, respectively. Group II rocks seem to have been generated by fractionation or metasomatism in each area. The 65-55 Ma gap between the two magmatic episodes implies a change in the tectonic environment from compression to extension.